Sometimes the objective rational choice is to do nothing for the mega-disasters and instead just prepare/harden for once-a-century or once-a-millennium event.
The Carrington event in 1859 caused serious damage to telegraphs, for instance.
It's a risk that is more once a century than once a millenium... so we may very possibly see one this century.
Considering how reliant we are on electricity, if everything goes down for more than a couple of days at most there is bound to be extremely serious civil unrest on a global scale: just imagine no access to money and thus not being able to buy food, fuel, etc. and then supply chains shutting down anyway.
... but the specific strength does come into play when discussing the costs of hardening and mitigation.
This event is twice as powerful as the previously known largest event.
This should not surprise us, by the way. I'd expect major solar storms to follow a Zipf's law distribution: https://en.wikipedia.org/wiki/Zipf%27s_law
That means that the power is inversely proportional to the frequency. This is common. You see it in word frequency, river length, population sizes of cities, etc.
So over a 15,000 year period, the biggest solar flare should be about 2x as big as the next biggest. And random Miyake events, which we've had 9 of over roughly 50,000 years, should be about 10x as big as a once every 160 year event like the Carrington event probably was.
All very rough, of course. But it may help give some intuition.
Forget about civil unrest. Chain of command in the military goes down and every unit is on it for themselves. Imagine generals not knowing if the President is still alive.
Or less dividends. This solar storm is a very extreme example, but we've all witnessed how greed got us very unprepared for a pandemic, climate changed induced weather events, new wars, etc.
Now, there's plenty to be said about the funding side of the equation to improve wealth inequality and/or increase budgets, but that's orthogonal to the spending-side necessary tradeoffs to harden against extremely rare disasters.
In other words, supposing you had the political pull to get more money entirely from the richest 0.01%, 1%, or 5% (and putting aside that not everyone agrees that would be good), that extra money would probably still be better spent on more infrastructure and social programs, not on hardening against extremely rare events.
Planetary GDP is around $85tn annually.
The US military budget on its own is around $1tn.
One-off mitigation costs followed by annual maintenance would barely be a rounding error on that.
They'd have the added benefit of hardening systems against hostile EMP, which is a more likely threat.
If I read you correctly, you're making an argument regarding the marginal effort to increase the budget by Y.
My argument is about the millions of things we could do with Y that would be more beneficial to human flourishing. Even if it's trivial to increase the budget by Y, that doesn't imply that it's trivial to increase the budget by Y enough times to run out of more important things to do.
Q: Are governments prepared to handle a solar storm 10 times as big as the Carrington Event?
A: Probably not, and that's not something we should fault them for or worry about.
Original (longer) argument:
It's possible that hardening against a Carrington-level event is a good use of money, but that's not the original question. The original question is about an event 10 times as big, twice as big as the previously known largest event.
If I understand your assertion, you're asserting that the money could easily be raised with little downside. Maybe it can be. For the sake of argument, I'll grant you that.
But my assertion is that it's not a question of if the money could be raised to harden against a 15k-year solar storm. My assertion is that even if the money were already in hand, hardening against such an extreme event is probably not worth the cost.
Whatever the budget is, it will be finite. Given present technology, whatever that budget is, it won't be sufficient to give all people worldwide a utopian lifestyle while fully funding all promising medical research, etc. Tradeoffs will need to be made. Hardening against a 15k-year event an order of magnitude more powerful than the Carrington event is probably not the best use of finite budget.
If I could start a side gig in a polka band and sell plasma to raise enough money to make my Florida home polar bear proof, that wouldn't have any bearing on the wisdom of polar-bear-proofing a home in Florida. Even assume that cost were the only downside to polar-bear-proofing a home, the availability of the money wouldn't be the problem with the plan. The problem is that the other things I could do with my limited resources would be much more beneficial. Yes, all else being equal, a polar-bear-proof house in Florida is better than a non-bear-proof house in Florida. In the real world, all else is not equal.
There is almost always a trade-off. Some risks aren't worth mitigating, and we're better off accepting them than either worrying ourselves over them or wasting limited resources on unwise mitigations.
My point is that a lack of political will to come up with the funding isn't the biggest obstacle to such extreme measures.
Perfect safety is a bad goal, and worrying that we aren't perfectly safe is unwise.
Edit: if I've missed something and you're talking about some hypothetical world where we've already solved cancer, heart disease, poverty, wealth inequality, climate change, polio, dementia, put basements / tornado shelters in all of the houses and trailer parks in Tornado Alley, figured out why the laundry always contains an odd number of socks, etc. Sure, then if we could scrounge up enough money to harden against a solar storm 10 times as powerful as the Carrington event, then go for it.